All fiber lines include some type of washing arrangement in order to separate the digestion liquor from the pulp. Later on in the process washing equipment is provided to separate bleaching liquors after bleaching stages. There exist a plurality of different types of washing equipment operating according to different principles.
One type of washing arrangement is the drum washer where the pulp is dewatered on a rotary drum after the addition of washing liquid, which displaces the liquor remaining in the pulp web after preceding process stages, for example a digestion stage or a bleaching stage. Another known washing arrangement is a washing press consisting of two counter-rotating perforated rolls. These types of washing arrangements have in common that the rotary drum or roll is often sealed at the ends from the pressure in a forming or washing zone against the surrounding atmospheric pressure. The sealing is often created by an overall end seal, which extends about either the entire or greater parts of the circumference of the rotary cylindrical element, i.e. the circumference of the drum or roll. The seal is often made of a polymer material. The seal is applied to the rotary roll or drum, for example, by water pressure, air pressure or spring force and acts as a mechanical seal. The maximum overpressure in the forming or washing zone is about 1 bar, which implies that the applied contact pressure usually is about 1.5 to 2 bar.
In washing arrangements of the aforesaid type the diameter of the drums or rolls is often relatively great, which means that a peripheral end seal, which extends about the entire circumference of the drum or roll or at least about the greater part thereof, can be up to 10 to 15 m long. In relation to the thickness and width of the seal the length is great, because the magnitude of thickness and width is about some tens of mms. The force required to hold the seal in place and to prevent co-rotation is significant. If the friction coefficient against the roll or drum is also high, the force required to hold the seal in a desired position is still greater. As a consequence of the significant force strain, the seal gradually elongates by creep to eventually fracture. The progress is often accelerated, in that the polymer material in the seal softens by the heat and ages by chemicals present in the liquors within the washing arrangement.
Owing to the elongation of the polymer seal, problems can arise with the means which apply the seal against the roll/drum, because its position can be disturbed. The application means can be, for example, a pressurized rubber hose, or spring element or a combination thereof. When, for example, the application means is an overall rubber hose, it can be subjected to shearing and is elongated, which can imply that fracture can occur if it lands outside its groove. When the seal is not enclosed at its ends, it can at its elongation land outside its groove and get into contact with rotating machine parts or be damaged in another way. The application means can also be spring elements, which during elongation of the seal can be displaced.
One of the objects of the present invention is to solve the aforesaid problems.